Process for the preparation of semiconducting ceramics composed of metal oxides, in particular of tin oxide, especially for varistors

ABSTRACT

Process for the preparation of a semiconducting ceramic based on doped tin oxide SnO 2  by a process of “PADO” (Precursor Alloy Direct Oxidation) type applied to an alloy of tin and of doping metals or by a process of “PADO” type applied to tin, the doping metals being added in the form of oxides to the powder subjected to sintering.

TECHNICAL FIELD

The invention relates to a process for the preparation of semiconductingceramics composed of metal oxides, more specifically of semiconductingceramics, composed of one or more base metal oxides, such as tin oxideSnO₂ and of one or more doping metal oxides.

Such semiconducting ceramics, whether bulk or else in the form of thinlayers, are used in particular to manufacture resistors which arenonlinear as a function of the voltage and in particular varistors,which are used, for example, in low-, medium- and high-voltage lightningarresters or voltage-limiting components associated, for example, withan item of electrical or electronic equipment.

The technical field of the invention can thus be defined very generallyas that of ceramics based on metal oxides and of their preparation andmore particularly as that of ceramics exhibiting a resistance which isnonlinear as a function of the voltage, such as varistors.

Resistors which are nonlinear as a function of the voltage, such asvaristors, based on silicon carbide, selenium rectifiers and p-njunction diodes made of silicon or germanium, have been widely used forthe voltage stabilization of electrical circuits or the suppression ofnormally high overvoltages induced in electrical circuits.

The electrical characteristics of such a nonlinear resistor are inparticular:

-   -   the nonlinearity coefficient α; the higher a is, the better the        performance of the material,    -   the maximum acceptable electric field before breakdown E_(s) (in        V/mm); the higher it is, the better the performance of the        material. This field E_(s) corresponds to a voltage before        breakdown V_(s).

Currently, resistors are essentially composed of sintered bodies of zincoxide ZnO optionally comprising one or more additives or doping agentschosen, for example, from metal oxides.

Thus, the documents FR-A-2 174 174, FR-A-2 174 175, FR-A-2 174 176 andFR-A-2 194 026 disclose varistors formed of a sintered body essentiallycomposed of ZnO with, as additives, Bi₂O₃, Sb₂O₃ and Co₃O₄.

However, it was suggested, in 1973 and then in 1974, to employ tin oxideSnO₂ in the manufacture of varistors, first of all as doping agent andthen as main oxide constituting the ceramic.

The document JP-A-49 105196 (1974) discloses an improvement in theelectrical properties of varistors made of ZnO by doping a mixture ofZnO, NiO₂, BaO and TiO₂ with 1 mol % of SnO₂; this mixture issubsequently compacted and sintered at 1350° C. in air.

The documents JP-A-48 099695 (1973) and JP-A-49 041897 (1974) provideformulations which can be expressed, for example, by: Bi₂O₃ 7 parts;(ZnO)_(0.87)(SnO₂)_(0.12)(YF₃)_(0.01) 100 parts, and which make itpossible to achieve electrical characteristics of varistors which arehighly varied, simply by modifying the Bi₂O₃ content.

Thus, values of the nonlinearity coefficient α of 10 to 51.3 andvoltages V_(s) of 235 to 889 V are obtained.

The documents JP-A-49 108590 (1974) and JP-A-49 047898 (1974) provide,for the first time, compositions for varistors predominantly based onSnO₂ instead of ZnO, although the latter oxide is, however, stillpresent in minor amounts. These compositions comprise SnO₂ (up to 70 mol%), ZnO and Sb₂O₃ doped with Bi₂O₃, V₂O₅, Nb₂O₅, Cr₂O₃ or Mn₂O₃.

These compositions make it possible to obtain high nonlinearitycoefficients, the maximum value indicated for the latter being 30.

Later, compositions for varistors based on SnO₂, this time completelydevoid of ZnO, are disclosed in the documents JP-A-49 129192 (1974) andJP-A-49 129193 (1974). These compositions are defined generally by thefollowing compositions as mol %: 40 to 99.85 of SnO₂, 0.05 to 30 ofSb₂O₃ and 0.1 to 50 mol % of CoO.

The ceramics prepared from a mixture composed of 99.9 mol % of SnO₂,0.05 mol % of Sb₂O₃ and 0.05 mol % of Bi₂O₃ or from a mixture composedof 99.85 mol % of SnO₂, 0.05 mol % of Sb₂O₃ and 0.1 mol % of CoO,compacted and sintered, have electrical properties of varistors whichare relatively mediocre, with α and V_(s) values respectively of lessthan 10 and 25 V.

However, it may be considered that varistors based on SnO₂ have onlyreally taken off since 1995, with the studies mentioned in the paper byS. A. Pianaro, P. R. Bueno, E. Longo and J. A. Varela, “A new SnO₂-basedvaristor system”, which describes in particular a varistor compositioncomprising, as mol %: 98.9% of SnO₂, 1% of CoO, 0.05% of Nb₂O₅ and 0.05%of Cr₂O₃.

This composition exhibits advantageous electrical properties, inparticular a nonlinearity coefficient α of 41 and a maximum electricalfield before breakdown E_(s) of 400 V/mm, which are associated withmonophase structures, the grain boundaries of which are apparentlydevoid of precipitated phases.

The document BR-A-96 00174-7 discloses metal oxide compositions forvaristors composed essentially of tin oxide (SnO₂) doped with variousmetal oxides, such as cobalt oxide and niobium oxide.

A typical composition comprises from 97.5 to 99.45% of tin dioxide(SnO₂), from 0.5 to 2.0% of cobalt oxide (CoO) and from 0.05 to 0.3% ofniobium oxide (Nb₂O₅). These compositions are subsequently subjected tosintering at a temperature of 1300 to 1350° C. to give a ceramic.

As regards their process of preparation, the semiconducting ceramicmaterials for semiconducting ceramic pellets or varistors used forprotecting from overvoltages are generally prepared from theirconstituent oxides in the pulverulent form.

Thus, the semiconducting ceramic materials most widely used at themoment, which are based on ZnO, are prepared from pulverulent oxidescomposed of the predominant oxide, which is ZnO, and doping oxides, suchas nickel, chromium, manganese, magnesium, bismuth, antimony, silicon orcobalt oxides, and the like.

Generally, the conventional chemical processes for the preparation ofceramic materials thus consists in weighing the constituent oxides, inmixing them and in milling them, and in then forming a mixture in anaqueous medium in order to obtain a slip.

This slip is atomized and dried (spray drying) to form agglomerates of afew hundred microns, which are subsequently shaped by pressing and thensintered at high temperature.

Finally, metal electrodes are deposited and the component is coated, onits other surfaces, with a material which provides electrical insulationand physicochemical and mechanical protection.

However, this type of process is complex to carry out and requires largemilling and heating devices. Furthermore, it is difficult to obtain goodchemical homogeneity as the intimate mixing of the milled components, atbest micrometric in size, can never be perfect.

It is in order to overcome the disadvantages of the conventionalprocesses for the preparation of ceramics, in particular ofsemiconducting ceramics for varistors, described above and in particularin order to obtain a powder which is homogeneous at the molecular levelformed of alloyed metal oxides that the process referred to as the“PADO” or Precursor Alloy Direct Oxidation process has been provided.

The PADO process is disclosed in the document FR-A-2 674 157 and, withsome variations in comparison with the document FR-A-2 674 157, in thedocument EP-A1-0 580 912 and in the document U.S. Pat. No. 5,322,642.

In the PADO process, the base components or starting materials forproducing the powder intended to give the semiconducting ceramic are nolonger metal oxides but alloys or mixtures of metals which are onlysubsequently oxidized, either in the solid phase or in the liquid phaseor in the vapour phase.

The PADO process, disclosed, for example, in the document FR-A-2 674157, comprises the following successive steps:

-   -   placing the various chosen metals in a crucible;    -   melting the said metals under a reducing atmosphere while mixing        the liquid for the purpose of homogenizing it;    -   casting the liquid alloy in an ingot mould in the said reducing        atmosphere;    -   allowing the ingot obtained to cool;    -   reducing the ingot to a powder with a predetermined particle        size, each of the particles of which is homogeneous;    -   oxidizing the said particles.

The ingot obtained can be reduced to a powder by milling or to chips bymachining, so that the ingot, the powder or the chips can be left underan appropriate atmosphere for the purpose of their oxidation.

It is also possible to melt the ingot to give a liquid alloy which issprayed for the purpose of obtaining solid fine components or grains ofhomogeneous composition which are subsequently oxidized.

Once the powder has been obtained, it is compacted in the form ofpellets by cold pressing, followed by sintering at high temperature.

According to claim 10 of this document, the product obtained by theprocess is composed of an alloy of zinc oxide and of doping productscomposed of oxides of at least some of the following metals: Ni, Cr, Mg,Mn, Bi, Sb, Si and Co (on page 5, line 32, it is copper which ismentioned); tin is not mentioned.

The document EP-A1-0 580 912 discloses more fully a process of “PADO”type for the manufacture of a homogeneous powder formed of metal oxidesfrom alloys of metals in which the following successive steps arecarried out:

-   -   placing, in a crucible, the various metals provided for in order        to form the alloy;    -   melting the said metals under a neutral or reducing atmosphere        while mixing the liquid for the purpose of homogenizing it;    -   recovering the liquid alloy;    -   obtaining, from the alloy, a powder with a predetermined        particle size, each of the particles of which is homogeneous;    -   oxidizing the said particles.

In order to prepare a ceramic, for example a semiconducting ceramic,based on metal oxides, the powder formed of oxides which is obtained iscompacted, for example in the form of a pellet, and the compactedproduct is sintered at a temperature of greater than or equal to 800° C.

This process applies in particular to the manufacture of semiconductorsbased on zinc oxide which are doped with oxides of Ni, Cr, Mg, Mn, Bi,Sb, Co (or Cu); again, tin is not mentioned.

The powder with a predetermined particle size can be obtained by directspraying of the liquid alloy recovered or else by casting the liquidalloy in an ingot mould in a neutral or reducing atmosphere, followed bymelting the ingot to give a liquid alloy which is sprayed for thepurpose of obtaining solid fine components or grains of homogeneouscomposition.

The “PADO” process, for example disclosed in the document FR-A-2 674 157and in the document EP-A1-0 580 912, makes it possible, without havingrecourse to one or more milling and mixing stages, which are oftensources of pollution, to obtain a mixture formed of a perfectlyhomogeneous powder formed of oxides which can never be obtained byconventional processes employing large milling and heating devices. Theperfectly homogeneous powder obtained exhibits a homogeneity at amolecular level which had never been achieved until then.

However, the “PADO” process introduces an answer to the problem ofhomogeneity of the powders only in the context of the preparation ofpowders formed of oxides and then of semiconducting ceramicsspecifically based on zinc oxide (ZnO) and not in the case of ceramicsbased on other oxides, in particular of ceramics based on tin oxide(SnO₂).

This clearly emerges from the two documents mentioned above, where theonly ceramic mentioned is specifically a ceramic based on zinc oxidedoped with oxides, among which tin oxide is not mentioned.

There thus exist, from the viewpoint of the above, a need for a process,derived from the “PADO” process, which makes it possible to obtainpowders formed of oxides and then ceramics, these being perfectlyhomogeneous at the submicroscopic level, indeed even at the molecularlevel, that is to say with segregations which are as limited aspossible, which is not limited to powders and ceramics based on zincoxide and which applies in particular to powders and ceramics based ontin oxide (SnO₂).

There additionally exists a need for a process derived from the “PADO”process which makes it possible to obtain semiconducting ceramics basedon oxides exhibiting better electrical parameters, in particular α andE_(s) parameters, than those obtained with ZnO.

Finally, there additionally exists a need for ceramics based on oxidesexhibiting an increased density and consequently improved mechanical andthermal properties, in particular as regards their mechanical strengthand their ability to dissipate heat.

The aim of the present invention is to provide a process for thepreparation of ceramics based on oxides of metals which meets, interalia, the needs listed above.

The aim of the present invention is, in addition, to provide a processfor the preparation of ceramics based on oxides of metals which does notexhibit the disadvantages, limitations, failings and drawbacks of theprocesses of the prior art, in particular of the “PADO” process, andwhich solves the problems of the processes of the prior art.

These aims, and others also, are achieved, in accordance with a firstembodiment of the invention, by a process for the preparation of aceramic comprising, preferably composed of, a base metal oxide and atleast one doping metal oxide, in which the following successive stepsare carried out:

-   -   tin metal (Sn), and one or more (other) doping metals and/or one        or more salts of these (other) doping metals are placed in a        crucible;    -   the tin and the said doping metals and/or salts of doping metals        placed in the crucible are melted under a neutral or reducing        atmosphere while mixing the molten tin and doping metals and/or        salts of doping metals, in order to obtain a homogeneous liquid        mixture or alloy of metals;    -   starting from the said homogeneous liquid mixture or alloy of        metals, a powder formed of alloy of metals with a predetermined        particle size is prepared, directly or after an optional        operation of particle size classification, for example a sieving        operation, each of the particles of which is homogeneous;    -   all the said particles of the powder formed of alloy of metals        with a predetermined particle size or a predetermined particle        size fraction of the powder are completely or partially        oxidized, in order to obtain a homogeneous powder formed of        oxides of the metals which is completely or partially oxidized,        coalescence of the particles being avoided;    -   the homogeneous powder formed of oxides which is completely or        partially oxidized is shaped;    -   the shaped powder is sintered.

In this first embodiment, the process according to the invention can bedefined as a process of “PADO” type applied to tin and more specificallyto an alloy of tin and of doping metals and which is employed in orderto prepare ceramics formed of doped tin oxide.

The process according to the invention thus exhibits all the advantagesof the PADO process already mentioned above.

It was indicated above that it proved to be the case that the “PADO”process makes it possible to prepare oxide-based powders and ceramics,the grains of which are chemically homogeneous at the submicroscopicscale, indeed even molecular scale, only in the specific case of powdersand ceramics based on zinc oxide (ZnO).

The process according to the invention, in this first embodiment, is notdescribed in the documents of the prior art, where the preparation ofceramics based on SnO₂ by the PADO process is not mentioned. A fortiori,the preparation of ceramics based on SnO₂ comprising doping oxides,preferably specific doping oxides in specific proportions, is notdescribed either.

Nothing could allow it to be anticipated in the prior art that this PADOprocess could also be successfully employed to prepare powders and thenceramics based on tin oxide (SnO₂).

This is because the teachings which can be deduced from the successfuluse of the PADO process with zinc oxide (ZnO) cannot under anycircumstances be applied to tin dioxide (SnO₂) as very significantdifferences in behaviour exist between these oxides which in factrenders completely unpredictable and random the change from one oxide tothe other; zinc oxide exhibits, for example, a metal/oxygen ratio of 1,whereas, for tin, this ratio is equal to 2.

The success of the “PADO” process, proven only with zinc oxide, did notmean in any way that this process might be applied to tin oxide.

In addition, by virtue of the use of tin oxide SnO₂, the varistorsobtained exhibit better electrical characteristics than the varistors ofthe prior art, in particular obtained by the “PADO” process, for examplebetter α and E_(s) properties than those obtained with varistors basedon zinc oxide.

The electrical characteristics of the varistors prepared by the processaccording to the invention according to this first embodiment are alsobetter than those of the varistors based on tin dioxide of the documentsJP-A-49 108590, JP-A-49 047898, JP-A-49 129192, JP-A-49 129193, JP-A-05129106 and JP-A-05 129167, the α values of which are low and always lessthan 30.

To sum up, it may be indicated that the present invention relatesneither to the process for the preparation of an alloy nor to theatomization of the latter. The invention may be defined generally asbeing the application of the “PADO” process to SnO₂. The physicochemicalhistory of a powder is determining in the chemistry of the solid.Consequently, the results obtained with the process of the inventionwere unpredictable and absolutely uncertain as a person skilled in theart knows that the chemistry of tin is very different from that of zinc.

In the case where the oxidation of the particles of the powder formed ofalloy of metals is only a partial oxidation, this characteristic ofpartial oxidation of the metals or of the Sn metal (see below as regardsthe 2^(nd) embodiment) also fundamentally distinguishes the processaccording to the invention from the processes for the preparation ofsemiconducting ceramics of the prior art and in particular from theprocesses of “PADO” type, such as those disclosed, for example, in thedocuments FR-A-2 674 157 and EP-A-0 580 912, in which a completeoxidation is carried out.

In other words, in the process according to the invention, the powder tobe compacted is, when a partial oxidation has been carried out, acomposite composed of metal (metals) and of metal oxide(s).

Basically, in the process according to the invention and when a partialoxidation is carried out, the compacted powders are mixtures of metal(metals) and of metal oxides obtained by partial oxidation of the metalpowders.

For this reason, the raw ceramic (before sintering and after shaping)comprises tin metal and other metals, which is never the case in theprior art and which is not under any circumstances suggested by theprior art.

It may be said that, in this case, a metal-ceramic composite issubjected to sintering.

This partial and incomplete oxidation is a preferred characteristic ofthis first embodiment of the process according to the invention but alsoof the second embodiment (see below). It is admittedly mentioned inpassing in the document EP-A1-0 580 912 that the alloy powder may bepartially oxidized but, before compacting and sintering, this powder iscompletely oxidized, so that the ceramic before sintering does not underany circumstances comprise metal.

Surprisingly, it turns out that, by carrying out only partial oxidationof the alloy (or metal) powder, the presence of metal, for example oftin metal, in the raw ceramic makes it possible to increase thedensification of the ceramic after sintering. This is well observedexperimentally starting from metal particles with an initial size of theorder of 20 to 40 microns in diameter as the densifications arerelatively low. There is thus a change from approximately 75% toapproximately 85%.

The electrical characteristics of the varistors obtained (E_(s) and α)when recourse has been had to a partial oxidation are also excellent,for example better than those of the Japanese documents cited above.

When the partial oxidation gives a powder with 50% to 99.9% of oxide,the above effects are particularly marked.

A percentage of 64% of oxide in particular for tin gives the bestresults for the densification and the electrical properties.

The oxide or oxides of doping metals can be chosen from oxides ofcobalt, chromium, manganese, niobium, tantalum, transition metals, suchas Zn, and lanthanide metals.

Consequently, the other doping metal or metals placed in the cruciblecan likewise be chosen from the abovementioned doping metals and thesalts of metals placed in the crucible can likewise be chosen from thesalts of the abovementioned metals.

Preferably, the oxide or oxides of doping metals are chosen from cobalt,manganese, niobium and tantalum oxides.

More preferably, the ceramic comprises, as doping metal oxides,simultaneously, at the same time, cobalt oxide, manganese oxide, niobiumoxide and tantalum oxide.

The proportion by weight of the tin and of the other doping metal ormetals and/or salt or salts of doping metals placed in the crucible ispreferably such that it makes it possible to obtain a ceramic comprisinga proportion of tin oxide of greater than or equal to 90% by weight,preferably of greater than or equal to 95% by weight, more preferably ofgreater than or equal to 99% by weight, better still of greater than orequal to 99.995% by weight.

In other words, the proportion by weight of the other doping metal ormetals and/or salt or salts of doping metals placed in the crucible issuch that it makes it possible to obtain a ceramic comprising, ascomplement to 100% by weight of tin oxide, less than 10% by weight,preferably less than 5% by weight, more preferably less than 1% byweight and better still less than 0.005% by weight of oxides of dopingmetals.

This percentage of SnO₂ in the final ceramic being observed, theproportion of the other doping metal or metals and/or salt or salts ofdoping metals in the crucible is such that it makes it possiblepreferably to obtain a ceramic comprising, as complement to 100% byweight of tin oxide, one or more among the following oxides in thefollowing proportions by weight:

-   -   0.1 to 3% of cobalt oxide,    -   0.01 to 3% of chromium oxide,    -   0.01 to 3% of manganese oxide,    -   0.01 to 0.5% of niobium oxide,    -   0.01% to 0.5% of tantalum oxide,    -   0.01 to 0.5% of one or more transition metal oxides,    -   0.01 to 0.5% of one or more lanthanide metal oxides, such as        lanthanum oxide.

This is because the inventors have demonstrated that the PADO processwas employed with even better results in preparing ceramics based on tinoxide if the latter preferably received the addition of oxides ofspecific doping agents, these oxides being more preferably in thespecific proportions cited above.

In other words, the unexpected adapting of the “PADO” process toceramics based on SnO₂ is carried out optimally if specific dopingagents are preferably added to the ceramics.

This optimization is even better if these specific doping agents areadded in specific proportions, which thus constitutes a twofoldselection.

The fact that the “PADO” process can be applied to SnO₂ with resultswhich are further improved provided that, preferably, such doping agentsare selected and the fact that this improvement is even more obvious insuch specific ranges of contents are in no way mentioned in the priorart.

The addition of these specific doping agents, preferably in thesespecific proportions, further improves the electrical properties, whichare already intrinsically improved by the use of SnO₂ instead of ZnO,and the other properties, such as the density.

Oxides of doping agents which are particularly preferred are chosen fromcobalt, manganese, niobium and tantalum oxides, such as Co₃O₄, MnO₂,Nb₂O₅ and Ta₂O₅, preferably in the proportions mentioned above.

A preferred ceramic will comprise the addition of all four of thesedoping oxides, preferably in the proportions mentioned above.

For example, this ceramic will have the following composition by weight(as % by weight):

-   -   SnO₂: 98.24%;    -   Co₃O₄: 0.05%;    -   MnO₂: 1.69%;    -   Nb₂O₅: 0.01%;    -   Ta₂O₅: 0.01%.

Generally, the SnO₂ makes it possible to use a smaller number of dopingagents, these doping agents being used in amounts which are loweroverall and these doping agents in addition being chosen from dopingagents which are less polluting, less toxic and cleaner.

In comparison with the ceramics based on ZnO, the relative content ofdoping agents with respect to the base metal oxide or reference oxide isgenerally 10 times less (10 times less doping agents) in ceramics basedon SnO₂.

Thus, the % of doping agents is of the order of 2%, in particular 1.76%,in the above example, whereas it is generally of the order of 10% in thecase of ZnO formulations. Furthermore, toxic doping agents, such asantimony oxide, are preferably not employed.

The alloy powder with a predetermined particle size can be prepared bycooling, preferably rapidly or suddenly (quenching), the homogeneousliquid mixture or alloy of metals, so as to solidify it, while retainingthe chemical homogeneity of the mixture or alloy of liquid metals (hightemperature), and by then dividing the solidified homogeneous alloy ofmetals to give a powder formed of alloy of metals with a predeterminedparticle size.

The cooling of the homogeneous liquid mixture or alloy of metals so asto solidify or set it can be carried out by casting the liquid mixtureor alloy of metals in an ingot mould under a neutral or reducingatmosphere and by then cooling the ingot obtained.

The dividing of the solidified homogeneous alloy can be carried out inthe liquid phase by melting it again to give a homogeneous liquid alloyof metals which is sprayed or atomized by a stream of gas or of liquidand rapidly cooled (quenched).

The powder formed of alloy of metals with a predetermined particle sizecan also be prepared directly from the homogeneous liquid alloy ofmetals resulting from the second stage of the process by spraying oratomization with a stream of gas or of liquid and rapid cooling(“quenching”).

It is found that the homogeneity of the powder is improved by quenchingthe stream of metal in a liquid coolant.

The stream of gas can be a stream of neutral or reducing gas, such ashydrogen, nitrogen, argon or their mixtures.

The stream of gas can be a stream of oxidizing gas, such as air, airenriched in oxygen, or oxygen optionally enriched in water vapour.

The dividing of the solidified homogeneous alloy of metals can also becarried out in the solid phase by abrasion or milling.

The powder formed of alloy of metals can optionally be separated intoseveral particle size fractions.

The powder formed of alloy of metals may be completely oxidized, that isto say that the oxidized powder comprises 100% by weight of oxide.

Alternatively, the oxide powder may be only partially oxidized.

In this case, the powder formed of alloy of metals may be partiallyoxidized to a percentage of 50 to 99.90% by weight, preferably of 55 to80 or 85% by weight, more preferably of 60 to 70% by weight, that is tosay that the oxidized powder comprises from 50 to 99.90% by weight ofoxide, preferably from 55 to 80 or 85% by weight of oxide and morepreferably from 60 to 70% by weight of oxide.

Preferably, the powder formed of alloy of metals is partially oxidizedto a percentage of 64% by weight.

The complete or partial oxidation of the particles of the powder formedof alloy of metals with a predetermined particle size can be carried outby bringing the said particles into contact with an oxidizing gas from atemperature and/or for a period of time which is (are) sufficient toproduce a desired percentage of oxides of metals in the powder, forexample for the oxidation to be complete.

For example, in the case of tin, a residence of one minute at 900° C. issufficient to oxidize a monolayer of metal particles with a meandiameter of 20 to 40 microns.

These experimental conditions for the oxidation are to be specified by athermogravimetric analysis. For example, in the case of pure tin, atemperature of 900° C. for 15 minutes makes it possible to completelyoxidize the tin to give tin oxide.

In other words, a stationary temperature phase is observed, theparticles being maintained at this temperature for a period of timesufficient for the oxidation to be complete.

The degree of oxidation of the particles is, in the case of tin,controlled or conditioned essentially by the temperature of thestationary oxidation phase and not by its duration, as in the case ofzinc.

It was completely unexpected that, in the case of a mixture or alloybased on tin, mastery of the complete oxidation is obtained byessentially controlling the temperature of the stationary oxidationphase and not its duration, as is the case for zinc and its alloys.

This means that it is thus possible to obtain, for example, completeoxidation, whatever the duration of the stationary phase, on conditionof being positioned at the appropriate temperature.

The oxidizing gas can be chosen from gases comprising oxygen optionallyenriched in water vapour and/or in carbon dioxide, such as air, airenriched in oxygen, oxygen, or air enriched in carbon dioxide and/or inwater vapour; or mixtures of CO and of CO₂.

The operation of bringing the particles into contact with the oxidizinggas can be carried out at a temperature of greater than or equal to 400°C., for example from 400 to 950° C.

The duration of this contacting operation is generally from 6 hours to 1second, preferably from 4 hours to 2 seconds.

The contacting operation can thus be carried out, for example, for 4hours at 400° C. or 2 seconds at 900° C.

In the process according to the invention, in this first embodiment, theaim is to prepare a powder comprising oxides (case of partial oxidation)or composed of oxides (case of complete oxidation) which is finelydivided, which is homogeneous, which has segregations which are aslimited as possible, which is isotropic and which has a surfacecondition which has to facilitate the subsequent densification.

The powder has a particle size which is generally submicronic in meansize (for example, mean diameter) of the particles.

The completely or partially oxidized homogeneous powder can be shaped bycompacting it in the form of pellets, for example by cold pressing.

The sintering can be carried out at a temperature of 1100 to 1350° C.for a period of time generally of greater than or equal to 30 minutes,preferably from 30 minutes to 2 hours.

The above aims and yet other aims are achieved, in accordance with asecond embodiment of the process of the invention, by a process for thepreparation of a semiconducting ceramic comprising, preferably composedof, a base metal oxide and at least one doping metal oxide, in which thefollowing successive steps are carried out:

-   -   tin is placed in a crucible;    -   the tin is melted, so as to obtain liquid tin;    -   a tin powder with a predetermined particle size is prepared from        the liquid tin, directly or after an optional operation of        particle size classification (sieving);    -   all the tin powder with a predetermined particle size or a        predetermined particle size fraction of the powder is completely        or partially oxidized;    -   one or more doping metal oxide(s) is (are) added to the        partially or completely oxidized tin powder;    -   the partially or completely oxidized tin powder and the doping        metal oxide(s) are mixed;    -   the mixture of partially or completely oxidized tin powder with        the doping metal oxide(s) is shaped;    -   the shaped powder is sintered.

The doping metal oxide(s) added to the completely or partially oxidizedtin powder are chosen from cobalt, chromium, manganese, niobium andtantalum oxides, transition metal oxides, such as zinc oxide, and oxidesof lanthanide metals, such as lanthanum oxide.

Preferably, the oxide(s) of doping metals are chosen from cobalt,manganese, niobium and tantalum oxides.

More preferably, the ceramic comprises, as doping metal oxides of,simultaneously, at the same time, cobalt oxide, manganese oxide, niobiumoxide and tantalum oxide.

The doping metal oxide(s) are added to the partially or completelyoxidized tin powder in a percentage by weight such that it makes itpossible to obtain a ceramic comprising a proportion of tin oxide ofgreater than or equal to 90% by weight, preferably of greater than orequal to 95% by weight, more preferably of greater than or equal to 99%by weight and better still of greater than or equal to 99.995% byweight.

In other words, the doping metal oxide or oxides can thus be added tothe partially or completely oxidized tin powder in a percentage byweight such that it makes it possible to obtain a ceramic comprising, ascomplement to 100% by weight of tin oxide, less than 10% by weight,preferably less than 5% by weight, more preferably less than 1% byweight and better still less than 0.005% by weight of oxides of dopingmetals.

This percentage of SnO₂ in the final ceramic being observed, the dopingmetal oxide(s) are added to the partially or completely oxidized tinpowder in a percentage by weight such that it makes it possible toobtain a ceramic comprising, as complement to 100% by weight of tinoxide, one or more among the following oxides in the followingproportions by weight:

-   -   0.1 to 3% of cobalt oxide,    -   0.01 to 3% of chromium oxide,    -   0.01 to 3% of manganese oxide,    -   0.01 to 0.5% of niobium oxide,    -   0.01% to 0.5% of tantalum oxide,    -   0.01 to 0.5% of one or more transition metal oxides,    -   0.01 to 0.5% of one or more lanthanide metal oxides, such as        lanthanum oxide.

Oxides of doping agents which are particularly preferred are chosen fromcobalt, manganese, niobium and tantalum oxides, such as Co₃O₄, MnO₂,Nb₂O₅ and Ta₂O₅, preferably in the proportions mentioned above.

A preferred ceramic will comprise the addition of all four of thesedoping oxides, preferably in the proportions mentioned above.

For example, this ceramic will have the following composition by weight(as % by weight):

-   -   SnO₂: 98.24%;    -   Co₃O₄: 0.05%;    -   MnO₂: 1.69%;    -   Nb₂O₅: 0.01%;    -   Ta₂O₅: 0.01%.

All the advantages and effects related to the use of specific dopingagents preferably in specific proportions mentioned in the context ofthe first embodiment can be repeated in full for the second embodimentof the invention and in particular as regards the advantages and effectsprovided by the use of cobalt, manganese, niobium and tantalum oxides,preferably in specific proportions.

The metal powder, for example tin powder, with a predetermined particlesize can be prepared in the liquid phase by melting the bulk tin to givea liquid metal which is sprayed or atomized by a stream of gas or ofliquid and cooled, quenched.

The liquid tin can be sprayed or atomized by a stream of neutral orreducing gas, such as nitrogen.

The tin powder with a predetermined particle size can be prepared in thesolid phase by abrasion or milling.

The tin powder can be separated into several particle size fractions.

The tin powder may be completely oxidized to a percentage of 100%.

Alternatively, the tin powder may be only partially oxidized.

In this case, the tin powder may be oxidized to a percentage of 50 to99.90% by weight, preferably of 55 to 80 or 85% by weight, morepreferably of 60 to 70% by weight, that is to say that the oxidizedpowder comprises from 50 to 99.90% by weight of oxide, preferably from55 to 80 or 85% by weight of oxide, more preferably from 60 to 70% byweight of oxide; better still, the powder is oxidized to a percentage of64% by weight.

For example, in the case of tin, a residence of one minute is sufficientto oxidize a monolayer of metal particles with a mean diameter of 20 to40 microns at 900° C.

The tin powder can be partially or completely oxidized by bringing thesaid powder into contact with an oxidizing gas from a temperature and/orfor a period of time which is (are) sufficient to produce a desiredpercentage of tin oxide in the powder.

As has already been mentioned above for the first embodiment of theprocess of the invention, the control of the degree of oxidation is, inthe case of tin, essentially controlled by the temperature of thestationary phase and not by its duration, as in the case of zinc.

The operation in which the particles are brought into contact with theoxidizing gas can be carried out at a temperature of greater than orequal to 400° C., for example from 400 to 950° C., for a period of timeof 1 second to 6 hours, preferably of 2 seconds to 4 hours.

The oxidizing gas can be chosen from gases comprising oxygen optionallyenriched in water vapour and/or in carbon dioxide, such as air, airenriched in oxygen, oxygen, or air enriched in carbon dioxide and/or inwater vapour; or mixtures of CO and of CO₂.

Analogously to the first embodiment, the process according to theinvention in the second embodiment can be defined as a process of “PADO”type applied to tin which is employed in order to prepare ceramics basedon doped tin oxide, the doping metals being added in the form of oxidesto the powder subjected to the sintering and not in the form of metalsor of salts to the crucible at the beginning of the process.

All the arguments mentioned in the context of the first embodiment inconnection with the unexpected application of the “PADO” process to tincan be applied in full to the second embodiment of the process of theinvention.

The two embodiments of the process according to the invention arerelated by the same inventive concept, which is the surprisingapplication of the PADO process to tin in order to prepare ceramics forvaristors.

In addition, this second embodiment of the process of the inventioncomprises a specific sequence of steps, themselves specific, which isneither described nor suggested in the prior art.

First, in the process according to this second embodiment, use is madespecifically of pure bulk tin and not of a tin alloy, as in the priorart.

A partial or complete oxidation of the tin powder is then carried out.

The following step of the process, which consists in adding one or moredoping oxides to the partially or completely oxidized tin powder, isalso specific to the process according to the invention in thisembodiment.

It should be noted that the comments expressed above for the firstembodiment which relate to the use of an only partial oxidation of thealloy powder apply in full to the second embodiment in the case where apartial oxidation of the tin powder is carried out (and not of alloy asin the first embodiment). In particular, the powder subjected tosintering then comprises tin metal, tin oxide and the oxides of dopingmetals added.

In particular, similar improvements in the densification and in theelectrical properties are obtained.

The invention will be better understood on reading the detaileddescription which will follow given by way of illustration and withoutimplied limitation and made with reference to the appended drawing, inwhich:

FIG. 1 is a diagrammatic view in vertical section of an atomizationdevice for preparing a powder formed of alloy of metals with apredetermined particle size, each of the particles of which ishomogeneous.

The first embodiment of the process according to the invention will nowbe described in detail.

This process can be defined as being a “PADO” process adapted to aspecific alloy based on Sn.

In the first step of the process according to the invention, in itsfirst embodiment, tin, as base metal, that is to say as metal, the oxideof which is the base oxide of the ceramic to be prepared, and one ormore other doping metals mentioned above and/or more salts of thesedoping metals, preferably in the proportions defined above, are placedin a crucible or any other receptacle suitable for the melting ofmetals.

The term “base metal oxide” (this base metal oxide being, in the presentcase, tin oxide SnO₂) as used in the present description is usedgenerally to indicate that this oxide is predominant by weight in thefinal sintered ceramic, that is to say that this oxide generallyrepresents 50% by weight or more, preferably more than 50% by weight, ofthe final ceramic; the more preferred proportions of the base metaloxide (greater than or equal to 90, 95, 99 or 99.995% by weight) havebeen given above.

The term “doping agent” is a term commonly used by a person skilled inthe art in this field of the art.

The term “salt” is such as commonly used in inorganic chemistry andincludes chlorides, nitrates, and the like, but also in particularoxides.

The final ceramic can optionally comprise, in addition to the base metaloxide and the doping metal oxide, in particular, impurities and/or otheradditives. The term “impurities” is understood to mean substances whichoccur by chance, without being desired, in the ceramic and the term“additives” is understood to mean substances deliberately added to theceramic in order to obtain one or more specific properties. Preferably,the semiconducting ceramic is composed of a base metal oxide and atleast one doping metal oxide.

Subsequently, in a second step, the tin and the said doping metalsand/or salts of doping metals are melted under a neutral or reducingatmosphere, for example hydrogen, while mixing the molten metals inorder to obtain a homogeneous liquid mixture or alloy of metals (tin anddoping metals).

During this melting step, the salt or salts optionally present maydecompose, for example if they are nitrates. The possible contaminationcaused by this decomposition is very slight, as the actual result of thevery low content of doping agents.

This first step and this second step are conventional steps which caneasily be carried out by the man skilled in the art, for example usingthe device (oven) described in the document FR-A-2 674 157 (FIG. 1) andin the document EP-A1-0 580 912, to the description of which referencemay be made.

In the third step of the process in its first embodiment, a powderformed of alloy of metals with a predetermined particle size, each ofthe particles of which is homogeneous, is prepared from the saidhomogeneous liquid mixture or alloy of metals.

The said predetermined particle size can be obtained directly or else onconclusion of an optional sieving operation.

This powder formed of alloy of metals can be prepared directly byspraying or atomizing, without preliminary cooling, the liquid alloyprepared in the second step by a stream of gas or of liquid.

Use may be made, in carrying out the direct atomization of the liquidalloy, of the device described in FIG. 3 of the document EP-A1-0 580912.

Alternatively, it is possible, first of all, to cool the homogeneousliquid mixture or alloy of metals, so as to solidify it, and then todivide this solidified homogeneous alloy of metals to give a powderformed of alloy of metals with a predetermined particle size.

The cooling operation can be carried out by casting the liquid alloy ofmetals in an ingot mould under a neutral or reducing atmosphere and bythen cooling the ingot obtained, likewise under a neutral or reducingatmosphere.

Alternatively, the alloy, if it has been produced in a silica containersealed under vacuum comprising the various metals, this container havingbeen subsequently heated in an oven and regularly agitated in order toobtain a homogeneous liquid mixture, can finally be cooled by aquenching intended to solidify the alloy obtained.

The solidified homogeneous alloy of metals, such as an ingot, can bedivided by again melting this solid alloy (which is, for example, in theform of an ingot) in order to give a homogeneous liquid alloy of metalswhich is sprayed by a stream of gas or of liquid.

The liquid used for the atomization can be water.

The gas used for the atomization or spraying can be a reducing orneutral gas chosen, for example, from hydrogen, nitrogen, argon andtheir mixtures.

The gas used for the atomization or spraying can be an oxidizing gas,such as air, optionally enriched in oxygen and/or in water vapour, oroxygen, so that the fine particles of molten alloy are sprayed to givefine particles which are at the same time partially oxidized and cooled.

Preferably, in this embodiment of the invention, use is made of areducing or neutral gas; for this reason, fine particles or droplets ofalloys are then cooled and stored in a nonoxidized or very superficiallyoxidized metal state. In all cases and whatever the gas used for thespraying, in this embodiment of the process of the invention, the alloyparticles are necessarily subsequently subjected to complete or partialoxidation prior to shaping and sintering.

In the case where the gas used is an oxidizing gas, the spraying isgenerally carried out at a temperature of 400 to 1000° C., whereas, inthe case where the gas used is a neutral or reducing gas, the sprayingis generally carried out at a temperature of 230 to 1000° C.

The dividing, atomizing or spraying of the alloy can be carried out witha device such as that described in the documents FR-A-2 674 157 (FIG. 2)and EP-A-0 580 912 (FIG. 2), to the description of which reference maybe made.

The dividing, atomizing or spraying in the liquid phase can also becarried out by atomizing the mixture, alloy or liquid with a stream ofgas, for example with nitrogen gas, using the device described in FIG.1.

This device comprises three parts: a part intended for the melting ofthe alloy (or of the metal, in the case of the second embodiment of theinvention: see below); a chamber which is the atomizer proper, which isprovided at its base with a spray nozzle; and a tube or rod for blockingthe spray nozzle and for measuring the temperature at this nozzle.

The entire device is heated via electrical resistors and flushed withneutral gas, such as argon (other than the atomizing gas), in order toprotect the liquid from any premature oxidation which might promotesegregation of the oxides.

More specifically, the alloy or the metal (1) is melted in a receptacle(2) heated via a heating resistor (3) and equipped with a thermocouple(4). This receptacle is fitted into a branch connection (5) equippedwith a ground adapter (6) situated in the side wall (8) of the atomizer(7); the latter has the form of a substantially elongated verticalcylindrical chamber.

The adapter (6) makes it possible to rotate the receptacle (2)containing the alloy (or the metal) between two positions: a first“bottom” position (in solid lines), in which the alloy (or the metal) isheated and melted, and a second “top” position (in dotted lines), inwhich the molten liquid alloy (or metal) can be transferred into thechamber of the atomizer. The molten alloy is kept molten in the lowerpart of the chamber of the atomizer (7) by heating electrical resistors(9). A pipe (10) which emerges in the side wall of the chamber of theatomizer conveys a stream of inert gas (11), such as argon, into thechamber and prevents any oxidation of the molten alloy or metal.

A flow orifice (12) for the liquid alloy (or metal) occurs at the baseof the chamber of the atomizer, which orifice is blocked by a rod ortube (13) equipped at its centre with a thermocouple (14). When it isdesired to atomize or spray the molten alloy, the tube or rod (13) israised and a thin stream of liquid alloy (or metal) flows via theorifice into a nozzle (15). The latter is surrounded by an annularcavity (16) which receives a side feed of stream of atomizing gas, suchas nitrogen, via a pipe (17) also equipped with heating electricalresistors (18).

For this reason, the thin stream or flow of liquid alloy (or metal)which issues under the effect of gravity from the end of the nozzle issprayed or atomized by virtue of the partial vacuum produced by the gas,such as nitrogen, stream ring which surrounds the molten alloy flowingthrough the nozzle (15).

The droplets of liquid alloy or metal are subsequently cooled,preferably rapidly cooled, that is to say quenched, in order to recoveran alloy (or metal) powder. The solidified homogeneous alloy of metals(or metal), such as an ingot, can also be divided in the solid phase,for example by abrasion, milling or machining. This spraying oratomizing in the solid phase is generally carried out at the boilingpoint of nitrogen at 1 bar, namely −196° C.

On conclusion of the atomizing or spraying, the powder may alreadyexhibit the desired particle size; if not, it is subjected, for example,to a sieving operation.

In addition, the alloy powder (or metal powder, in the secondembodiment) can subsequently be separated into several particle sizefractions by sieving or any other separation process.

All the powder or only a predetermined particle size fraction, forexample the particle size fraction comprising the particles with adiameter of less than 40 μm, is subjected to the following stage of theprocess, which consists of a complete or partial oxidation of the alloypowder obtained above.

The term “complete oxidation” is understood to mean that the finalpowder obtained comprises 100% by weight of oxides.

The term “partial oxidation” is understood to mean that the final powdercomprises a proportion of oxide(s) of less than 100% by weight;preferably, the proportion of oxide(s) is 50 to 99.9% by weight;preferably it is from 55 to 80% or 85% by weight; more preferably it isfrom 60 to 70% by weight.

This complete or partial oxidation is generally carried out whileavoiding coalescence of the particles: this can be obtained by varyingthe physical parameters related to the technique used, such as thetemperature, the pressure, the rate, and the like, or by virtue of thetechnology employed: for example, use may be made of a fluidized,pulverulent bed.

This complete or partial oxidation is generally carried out by bringingthe alloy powder (or metal powder) into contact with an oxidizing gasfrom a temperature and/or for a period of time which is (are) sufficientfor the oxidation to be complete or for the desired percentage ofoxide(s) in the powder to be obtained.

For example, in the case of tin, a residence of one minute is sufficientto oxidize a monolayer of metal particles with a mean diameter of 20 to40 microns at 900° C.

The oxidizing gas can be any oxidizing gas suitable for this purpose butit is generally chosen from gases comprising oxygen optionally enrichedin water vapour and/or in carbon dioxide, such as air, air enriched inoxygen, oxygen, or air enriched in carbon dioxide and/or in watervapour; or mixtures of CO and of CO₂.

The operation in which the alloy (or metal) particles are brought intocontact with the oxidizing gas can be carried out at a temperatureranging from 60 to 1000° C. but is generally carried out at a hightemperature, namely a temperature of greater than or equal to 400° C.,for example from 400 to 950° C.

This contacting operation is carried out for a period of time sufficientfor the oxidation to be complete or for the desired percentage ofoxide(s) in the powder to be obtained. This period of time can be easilydetermined by a person skilled in the art, for using oxidation curvesfound during preliminary experiments, which give the exact amount ofalloy oxidized as a function of the temperature and of the period oftime.

The powder formed of oxide(s) which is obtained has a particle sizewhich is generally submicronic in mean size (for example, mean diameter)of the particles.

The powder formed of oxide(s) which is obtained is subsequently shapedin a known way, for example compacted in the form of pellets ofceramics, by cold pressing, for example by uniaxial cold pressing, usingan organic or inorganic binder, such as water.

The powder shaped or compacted, for example in the form of pellets, issubsequently sintered in a known way at high temperature, generally at atemperature of 1100 to 1350° C., for a period of time of greater than orequal to 30 minutes, preferably from 30 minutes to 2 hours, for exampleat 1350° C. for one hour, in order to densify it.

The densified ceramics obtained on conclusion of the process accordingto this first embodiment can be used in varistors, they being metallizedbeforehand.

The second embodiment of the process according to the invention will nowbe described in detail.

This process may be defined in particular as being a “PADO” processapplied to a pure tin and not to an alloy of tin and of doping agents.

For this reason, in a first step, pure bulk tin is provided, for examplein the form of lumps or shot or ingots.

In a second step, a tin powder with a predetermined particle size isprepared from the pure bulk tin.

The tin powder with a predetermined particle size can be prepared in theliquid phase by melting the bulk tin to give a liquid metal which issprayed or atomized with a stream of neutral or reducing gas, forexample chosen from hydrogen, nitrogen, argon and their mixtures, orelse with a stream of oxidizing gas.

The conditions of this atomizing or spraying in the liquid phase havebeen described in detail above in connection with the first embodiment;in particular, the device described in FIG. 1 can be used to carry outthis step.

The tin powder with a predetermined particle size can be prepared in thesolid phase by abrasion or milling under conditions already describedfor the first embodiment.

Likewise, it is possible, as in the first embodiment, to separate thetin powder into several particle size fractions.

All or only a predetermined particle size fraction, for example theparticle size fraction comprising the particles with a diameter of lessthan 40 μm, is subjected to the following step of the process, whichconsists of a partial or complete oxidation of the metal powder obtainedabove.

The definitions of and the conditions for the partial and completeoxidations have already been given above in the context of the presentembodiment.

The term “partial oxidation”, in contrast to the prior art, where acomplete oxidation is carried out, is understood to mean that only aportion of the said metal powder is oxidized and that a powdercomprising, for example, both tin dioxide and tin metal is obtained.

Preferably, the tin powder is partially oxidized to a percentage of 50to 99.9% by weight, preferably of 55 to 80% or 85% by weight, morepreferably of 60 to 70% by weight, that is to say that the oxidizedpowder comprises 50 to 99.90% by weight, preferably from 55 to 80% or85% by weight, more preferably from 60 to 70% by weight of oxide and theremainder of free metal, that is to say of free Sn.

More preferably, the tin powder is oxidized to a percentage of 64% byweight, that is to say that it comprises, for example, by weight, 64% ofSnO₂ and 36% of tin.

The partial or complete oxidation of the metal powder, for example ofthe tin powder, with a predetermined particle size is carried out bybringing the said powder into contact with an oxidizing gas at atemperature and for a period of time sufficient to obtain a desiredpercentage (for example, within the ranges defined above) of tin oxide,which percentage can reach 100% of tin oxide in the partially or totallyoxidized powder.

The gas used and the temperature conditions are analogous to thosedescribed above for the first embodiment of the process according to theinvention.

The contacting operation is carried out for a period of time sufficientto produce the desired percentage of oxide in the powder; this period oftime can be easily determined by a person skilled in the art, forexample using oxidation curves, as has been described above.

In the following step, one or more powdered doping metal oxide(s) is(are) added to the partially or completely, totally, oxidized tinpowder.

The oxide or oxides of doping metals added to the partially or totallyoxidized tin powder can be chosen from all suitable oxides of dopingmetals.

They are generally chosen from cobalt, chromium, manganese, niobium andtantalum oxides, transition metal oxides, such as zinc oxide andlanthanide metal oxides, such as lanthanum oxide. The oxides can becommercially available oxides or oxides prepared by any known process,indeed even by the “PADO” process.

The oxide or oxides of doping metal are added to the completely orpartially oxidized tin powder in a percentage by weight such that itmakes it possible to obtain a ceramic comprising, as complement to 100%by weight of tin oxide, the desired percentage of the doping metaloxides.

For example, the doping oxide or oxides can be added in a percentage byweight such that they make it possible to obtain a ceramic comprising,as complement to 100% by weight of metal oxide, for example of tinoxide:

-   -   0.1 to 3% of cobalt oxide, and/or    -   0.01 to 3% of chromium oxide, and/or    -   0.01 to 3% of manganese oxide, and/or    -   0.01 to 0.5% of niobium oxide, and/or    -   0.01% to 0.5% of tantalum oxide, and/or    -   0.01 to 0.5% of one or more transition metal oxides,    -   0.01 to 0.5% of one or more lanthanide metal oxides, such as        lanthanum oxide.

The powders are mixed using known processes. The shaping and sinteringstages are carried out under the same conditions as for the firstembodiment, optionally with slight adjustments.

The invention will now be described with reference to the followingexamples, given by way of illustration and without implied limitation.

Example 1

This example illustrates the first embodiment of the process accordingto the invention, in which the “PADO” process is adapted in order toprepare tin oxide doped with oxides of certain metals in specificproportions.

In this example, a ceramic is prepared which has the followingcomposition by weight (as % by weight):

-   -   SnO₂: 98.24%;    -   Co₃O₄: 0.05%;    -   MnO₂: 1.69%;    -   Nb₂O₅: 0.01%;    -   Ta₂O₅: 0.01%.

The “PADO” process is carried out on the five starting metals: tin andthen cobalt, manganese, niobium and tantalum as doping agents.

A liquid alloy is first prepared from these five metals. It issubsequently quenched in order to set the composition of the alloyobtained.

The alloy is subsequently placed in an atomizer, where it is sprayed bya stream of nitrogen gas to give an alloy powder.

The alloy is prepared and is atomized on an experimental laboratoryprototype.

Thus, the alloy is prepared in a silica container sealed under vacuum,which receives the various solid metals in the desired proportions. Thiscontainer is subsequently heated in an oven at 1100° C. and regularlyagitated in order to produce a homogeneous liquid mixture.

The molten alloy is then atomized, which carries out the quenchingintended to solidify the alloy obtained so as to retain the homogeneitythereof acquired at high temperatures.

The alloy is atomized or divided to give a powder in an atomizer oratomizing device, such as that described in FIG. 1.

The particle size fraction of the particles with a diameter of less than40 μm is separated from the whole of the powder for the continuation ofthe process.

The powder corresponding to this fraction is subsequently completelyoxidized to give an oxide powder; the complete oxidation is carried outby placing the alloy powder in an oven under an atmosphere of air and bymaintaining it at a temperature of 900° C. for a period of time of 5minutes.

Finally, this resulting powder is shaped under a pressure of 3 tonnes ina die equipped with two moving cylinders and then sintered at 1350° C.for 2 hours with a rise and a fall in temperature of 3° C./minute inorder to result in a bulk ceramic.

The electrical characteristics of the ceramics obtained aftermetallization are as follows: the nonlinearity coefficient α is 47(measured around 10⁻³ A/cm²) and the threshold field E_(s) is 450 V/mm(measured for 10⁻³ A/cm²).

Example 2

This example illustrates the second embodiment of the process accordingto the invention, in which the “PADO” process is modified by carryingout a partial (and no longer complete) oxidation of a tin powder (and nolonger of an alloy powder) and in which the doping oxides are added tothe oxidized powder, in this case partially oxidized powder, before thesintering.

In this example, a ceramic is prepared which has a composition by weightsimilar to that of Example 1:

-   -   SnO₂: 98.3%;    -   Co₃O₄: 0.05%;    -   MnO₂: 1.69%;    -   Nb₂O₅: 0.01%;    -   Ta₂O₅: 0.03%.

The “PADO” process is carried out on a starting product which is purebulk tin. The latter is first atomized under nitrogen in order toproduce a tin metal powder. The main parameters relating to theatomizing gas are a pressure of 3 bar, a flow rate of 45 l/minute and atemperature of 700° C. The atomizing device described in FIG. 1 isemployed.

The powder is sieved, so as to retain only the particle size fraction ofless than 40 μm. This powder is subsequently partially oxidized to 64%by weight, that is to say that it comprises, by weight, 64% of tindioxide SnO₂ and 36% of tin.

This partial oxidation is carried out by placing the tin powder in anoven under an atmosphere of air, by raising the temperature at the rateof 3° C./minute up to 750° C. and by observing a stationary phase atthis temperature for a duration limited to 5 minutes, before quenchingwith air.

The doping oxides (Co₃O₄, Nb₂O₅ and Ta₂O₅) in the powder form are thenadded in the desired amounts to the metal-ceramic powder.

This combined product is subsequently mixed and then shaped and sinteredunder the same conditions as Example 1 to give a ceramic.

The presence of tin metal in the raw ceramic, that is to say beforesintering, made it possible to increase the densification of the ceramicafter sintering from 75.2 to 85.4%.

The electrical characteristics of the ceramics obtained aftermetallization are as follows: the nonlinearity coefficient α is 53(measured around 10⁻³ A/cm²) and the threshold field E_(s) is 308 V/mm(measured for 10⁻³ A/cm²).

1. A process for preparing a substantially homogenous semiconductingceramic composition comprising base metal oxide and at least one dopingmetal oxide, the process comprising: a) melting and mixing under aneutral or reducing atmosphere said base metal oxide and doping metalsand/or salts of doping metals, in order to obtain a homogeneous liquidmixture; b) preparing a powder from said homogenous liquid mixture witha predetermined particle size, directly or after an optional operationof particle size classification, each of the particles of said powderbeing homogeneous; and c) completely or partially oxidizing said powderto obtain a homogeneous powder which is completely or partiallyoxidized, thereby preparing said substantially homogenous semiconductingceramic composition comprising a base metal oxide and at least onedoping metal oxide.
 2. The process according to claim 1, wherein theoxide or oxides of doping metals are selected from the group consistingof oxides of cobalt, chromium, manganese, niobium, tantalum, andtransition metals.
 3. A process according to claim 1, wherein the oxideor oxides of doping metals are selected from the group consisting ofcobalt, manganese, niobium and tantalum oxides.
 4. A process accordingto claim 1, wherein the doping metal oxides are selected from the groupconsisting of cobalt oxide, manganese oxide, niobium oxide and tantalumoxide.
 5. A process according to claim 1, wherein the proportion byweight of the doping metal or metals and/or salt or salts of dopingmetals in step a) is selected from the group consisting of 90% byweight, 95% by weight, 99% by weight, and 99.995% by weight.
 6. Aprocess according to claim 1, wherein the doping metal or metals and/orsalt or salts of doping metals in step a) comprises one or more amongthe following oxides in the following proportions by weight: 0.1 to 3%of cobalt oxide, 0.01 to 3% of chromium oxide, 0.01 to 3% of manganeseoxide, 0.01 to 0.5% of niobium oxide, 0.01% to 0.5% of tantalum oxide,or 0.01 to 0.5% of one or more transition metal oxides, 0.01 to 0.5% ofone or more lanthanide metal oxides, such as lanthanum oxide.
 7. Aprocess according to claim 1, wherein the said doping metal or metalsand/or salt or salts of doping metals in step a) comprises the followingoxides in the following proportions by weight: SnO₂: 98.24%; Co₃O₄:0.05%; MnO₂: 1.69%; Nb₂O₅: 0.01%; or Ta₂O₅: 0.01%.
 8. A processaccording to claim 1, wherein the powder of step b) is prepared bycooling, optionally rapidly, the liquid mixture of step a).
 9. A processaccording to claim 8, wherein step a) further comprises casting theliquid mixture in an ingot mould under a neutral or reducing atmosphereprior to the cooling of step b).
 10. A process according to claim 1,wherein step b) comprises spraying or atomizing by a stream of gas or ofliquid, and quenching quenched.
 11. A process according to claim 9,wherein step b) comprises spraying or atomizing with a stream of gas orof liquid, and quenching.
 12. A process according to claim 10 or 11,wherein in the homogeneous liquid is sprayed or atomized with a streamof neutral or reducing gas, such as hydrogen, nitrogen, argon or theirmixtures.
 13. A process according to claim 10 or 11, wherein thehomogeneous liquid is sprayed or atomized with a stream of oxidizinggas, such as air, air enriched in oxygen, or oxygen optionally enrichedin water vapour.
 14. A process according to either one of claims 8 and9, wherein the powder of step b) is prepared by abrasion or milling. 15.A process according to claim 1, wherein the powder of step b) isseparated into several particle size fractions.
 16. A process accordingto claim 1, wherein the powder in step c) is partially oxidized to apercentage selected from the group consisting of 50 to 99.9% by weight,55 to 80 or 85% by weight, and 60 to 70% by weight.
 17. A processaccording to claim 1, wherein the powder in step c) is oxidized to apercentage of 64% by weight.
 18. A process according to claim 1, whereinthe partial or complete oxidation of step c is carried out by bringingthe said particles into contact with an oxidizing gas from a temperatureand/or for a period of time which is (are) sufficient to produce adesired percentage of oxides of metals in the powder, for example forthe oxidation to be complete.
 19. A process according to claim 18,wherein the oxidizing gas comprising oxygen is optionally enriched inwater vapour and/or in carbon dioxide.
 20. A process according to claims18 or 19, wherein the oxidation of step c) is carried out at atemperature of greater than or equal to 400° C., for a period of time of1 second to 6 hours.
 21. A process according to claim 1 furthercomprising shaping the product of step c) by compacting it in the formof pellets.
 22. A process according to claim 21, wherein the shaping ofstep d) is carried out at a temperature of 1100 to 1350° C. for a periodof time of greater than or equal to 30 minutes.
 23. A process for thepreparation of a semiconducting ceramic comprising a base metal oxideand at least one doping metal oxide the following successive steps: tinis placed in a crucible; the tin is melted, so as to obtain liquid tin;a tin powder with a predetermined particle size is prepared from theliquid tin, directly or after an optional operation of particle sizeclassification; all the tin powder with a predetermined particle size ora predetermined particle size fraction of the powder is completely orpartially oxidized; one or more doping metal oxide(s) is (are) added tothe partially or completely oxidized tin powder; the partially orcompletely oxidized tin powder is mixed with the doping metal oxide(s);the mixture of partially or completely oxidized tin powder and of thedoping metal oxide(s) is shaped; the shaped powder is sintered.
 24. Aprocess according to claim 23, wherein the doping metal oxide or oxidesadded to the completely or partially oxidized tin powder are from thegroup consisting of cobalt, chromium, manganese, niobium and tantalumoxides and oxides of transition metals.
 25. A process according to claim23, wherein the oxide or oxides of doping metals are selected from thegroup consisting of cobalt, manganese, niobium and tantalum oxides. 26.A process according to claim 25, wherein the oxide or oxides of dopingmetals are selected from the group consisting of doping metal oxides,cobalt oxide, manganese oxide, niobium oxide and tantalum oxide.
 27. Aprocess according to claim 23, wherein the tin oxide is added in aproportion of greater than or equal to a percentage selected from thegroup consisting of 90% by weight, 95% by weight, 99% by weight, and99.995% by weight.
 28. A process according to claim 27, wherein thedoping metal oxide(s) are added to the partially or completely oxidizedtin powder in a percentage selected from the group consisting of: 0.1 to3% of cobalt oxide, 0.01 to 3% of chromium oxide, 0.01 to 3% ofmanganese oxide, 0.01 to 0.5% of niobium oxide, 0.01% to 0.5% oftantalum oxide, 0.01 to 0.5% of one or more transition metal oxides, and0.01 to 0.5% of one or more lanthanide metal oxides.
 29. A processaccording to claim 28, wherein the doping metal oxides are added to thepartially or completely oxidized tin powder in a percentage by weightsuch that it makes it possible to obtain a ceramic comprising, ascomplement to 100% by weight of tin oxide, the following oxides in thefollowing proportions by weight: SnO₂: 98.24%; Co₃O₄: 0.05%; MnO₂:1.69%; Nb₂O₅: 0.01%; or Ta₂O₅: 0.01%.
 30. A process according to claim23, wherein the tin powder with a predetermined particle size isprepared in the liquid phase by melting bulk tin to give a liquid metalwhich is sprayed or atomized by a stream of gas or of liquid and cooled,quenched.
 31. A process according to claim 23, wherein the liquid metalis sprayed or atomized by a stream of neutral or reducing gas.
 32. Aprocess according to claim 23, wherein the tin powder with apredetermined particle size is prepared in the solid phase by abrasionor milling.
 33. A process according to claim 23, wherein the tin powderis partially oxidized to a percentage selected from the group consistingof 50 to 99.9% by weight, 55 to 80 or 85% by weight, and 60 to 70% byweight.
 34. A process according to claim 23, wherein the tin powder isoxidized to a percentage of 64% by weight.
 35. A process according toclaim 23, wherein the tin powder is separated into several particle sizefractions.
 36. A process according to claim 23, wherein the tin powderwith a predetermined particle size is partially or completely oxidizedby bringing the said powder into contact with an oxidizing gas from atemperature and/or for a period of time which is (are) sufficient toproduce a desired percentage of tin oxide in the powder.
 37. A processaccording to claim 36, wherein the operation in which the particles arebrought into contact with the oxidizing gas is carried out at atemperature of greater than or equal to 400° C., for a period of time of1 second to 6 hours.
 38. A process according to claim 36 or 37, whereinthe oxidizing gas is chosen from gases comprising oxygen optionallyenriched in water vapour and/or in carbon dioxide.
 39. A processaccording to claim 23, wherein in which the mixture of the partially ortotally oxidized tin powder and of the doping metal oxide(s) is shapedby compacting it in the form of pellets.
 40. A process according toclaim 23, wherein the sintering is carried out at a temperature of 1100°C. to 1350° C. for a period of time of greater than or equal to 30minutes.
 41. The composition produced according to the process ofclaim
 1. 42. The composition produced according to the process of claim23.